In a wireless communications system such as AMPS, FM (frequency modulation) is used for communicating voice signals together with a supervisory audio tone (SAT) which is transmitted by a base station with a nominal frequency selected from three possible nominal frequencies of 5970, 6000, and 6030 Hz, and an FM deviation of .+-.2 kHz.+-.10%. SAT components of the signal transmitted by the base station are transponded by a mobile terminal back to the base station using a phase locked loop (PLL). The base station is required to detect the presence and loss of SAT within attack and release times, respectively, of 200 ms. The SAT is required to be present all the time for the voice channel, and voice calls can be interrupted if a valid SAT is not detected. Accordingly, reliable SAT detection is important for proper operation of the system.
In practice, reliability of SAT detection can be of concern in situations where there is a low C/I (signal carrier to interference) ratio, for example under co-channel interference (CCI) conditions, and situations where the mobile terminal may not fully comply with specifications. For example, in fading and shadowing conditions, the PLL in the mobile terminal may not be able to track well, so that the transponded SAT can drift in frequency. In addition, the FM deviation of the transponded SAT frequency may be different from the specified .+-.2 kHz.+-.10%, also adversely affecting SAT detection by the base station.
A digital PLL can be used to detect a sinusoidal signal, but is not well suited to SAT detection because it requires a relatively long capture time and does not work well in the fading channel environment of wireless communications systems. A matched filter or coherent detection technique can be used, but this only works well with SAT frequencies that are very accurate. The SAT detection performance of matched filters is seriously degraded for small frequency differences for example of 3 Hz, so that matched filters can not be used for detecting SAT from out-of-specification mobile terminals. Auto-correlation based SAT detectors have been used because they are not sensitive to SAT frequency variation, but they do not provide an optimal SAT detection.
McPherson et al. U.S. Pat. No. 4,698,769, issued Oct. 6, 1987 and entitled "Supervisory Audio Tone Detection In A Radio Channel", describes a base station SAT detector in which a DFT is applied to complex numbers derived by accumulation from samples of the received signal, and the powers at the SAT frequencies are compared with threshold levels to determine whether or not the respective SAT is present in the received signal. This patent describes the use of an 8-point DFT with an approximation used to simplify the calculations, but in practice a larger and more accurate DFT is required to provide a desired frequency resolution for SAT detection. Typically, a 32-point DFT may be desired. The resources required for computing a 32-point DFT are considerably greater than those required for computing an 8-point DFT.
Block computation of a 32-point DFT in known manner also creates a high peak digital signal processing (DSP) load for computing the DFT periodically, with relatively low DSP loads at other times. In practice in a base station of a wireless communications system it is desirable to avoid such high peak DSP loads.
Accordingly, an object of this invention is to provide an improved method of and apparatus for DFT computation for tone detection, particularly for detecting SAT in a wireless communications system.